1. Field of the Invention
The present invention relates to an antenna device, a feed element, and a communication terminal device, which are used in communication systems of HF-bands and UHF-bands.
2. Description of the Related Art
In 13.56 MHz-band RFID installed in a cellular phone terminal and employed for, e.g., NFC (Near Field Communication), an RFID IC chip and a matching element are generally mounted on a printed wiring board in many cases, and an antenna is affixed to the inner side of a terminal casing. Furthermore, the RFID IC chip and the antenna are electrically connected (DC-connected) through a spring pin, for example. However, such a connection method has the problem that a contact failure may occur due to friction in a contact portion or a positional deviation caused in assembly.
In view of the above-mentioned problem, a reader/writer is disclosed which has a structure including an antenna substrate on which a loop antenna is disposed, and a control substrate on which a transmitting and receiving circuit is disposed (see Japanese Patent No. 4325621). In the structure disclosed in Japanese Patent No. 4325621, a coil disposed on the control substrate and the loop antenna are magnetically coupled to each other to establish electrical connection of the loop antenna with the transmitting and receiving circuit. As a result, a physical contact portion is not present, and the occurrence of a contact failure can be avoided.
In the structure disclosed in Japanese Patent No. 4325621, however, because of utilizing the magnetic coupling, a coupling degree changes due to a variation in the mounted position of the coil, and the distance between a surrounding component and the coil varies due to a variation in the mounted position of the coil, whereby a value of stray capacitance generated between the surrounding component and the coil is changed. This causes a problem that antenna characteristics vary.